Portland cement production is an industry producing 3,400 million tonnes of cement powder per annum, and is the second largest source of man-made CO2 emissions, with approximately 0.8 tonne of CO2 produced per tonne of cement. Of this, about 60% derives from the CO2 emitted from processing limestone CaCO3 to lime CaO in the production of cement clinker in a process known as calcination, and 40% arises from the burning of fossil fuels to produce the cement. The reduction of CO2 emissions to reduce global warming is required, and the Portland Cement industry is under pressure to reduce its CO2 emissions.
Incremental improvements to the efficiency of the production process are one approach, but substantial reductions are not possible by incremental improvements. The other approaches that have been developed include the application of post-combustion capture in which the CO2 from the exhaust gas stream, which contains the CO2 from both carbonate calcination and fuel combustion. Established processes, such as amine stripping, are too capital expensive, and the recent focus has been on using lime, CaO, as a high temperature CO2 sorbent, in a process called Calcium Looping. This process is at the pilot stage of demonstration. It has the advantage that the spent CaO sorbent is consumed in the Portland cement. This process has the disadvantage that the capture process is carried out at about ambient pressure and the size and cost of the plant would be very large, approaching that of the Portland cement plant itself. A significant concern is the penalty arising from the consumption of additional energy for the Calcium Looping Process. This is a cost, and adds to the scale of the plant.
Another approach is Oxy-fuel combustion in which pure oxygen is used for combustion instead of air, in which case the exhaust gas is CO2 and steam, which allows the CO2 to be captured by condensing the steam. The cost of a cryogenic separation plant is a very large cost, and the Portland cement plant has to be significantly redesigned to account for the very different flue gas flows through the kilns and another processes.
Portland cement plants typically use coal and waste materials as a fuel, rather than natural gas, so that approaches based on pre-combustion capture to produce a hydrogen gas stream from natural gas are generally not applicable.
Lime production is similar to that of Portland cement, except that a higher quality of limestone is used, and sand and clay is excluded to produce a lime product
There is a need for a process that can significantly reduce the CO2 emissions from a Portland cement or lime process without the requirement for large additional processing plants described above. In all the CO2 reduction schemes considered above, the CO2 must be compressed for sequestration.
Portland cement production now uses the “dry process” in which lime and sand particles fuse in the rotary kiln, compared with the “wet process” previously used in which the limestone, sand and other additives are pressed into a pellet. The dry process has a lower energy demand than the wet process. This invention is directed to the dry process.
In the dry process, limestone is received as rocks, which are crushed and ground to a particle size of less than 100 microns, and uniformly mixed with sand that has also been ground to less than 100 microns. Other ground materials, such as clay and iron oxide may be added for a particular cement formulation. Generally, the different particles streams are mixed in a hopper designed for efficient mixing to give a homogenous mixture. The dry cement process relies on an efficient mixing to promote fusion and reaction in a rotary kiln.
In the conventional dry process, the mixed powder is pre-heated by the flue gas exhaust from kiln using a pre-heater cyclone stack, which is a bank of cyclones in series. In each stage, the colder particles are heated by mixing with the hotter flue gas steams, and the equilibrated gas and particles are separated in a cyclone. This process is repeated many times in a sequence in which the temperature of the particles are raised and that of the gas is reduced. A modern plant may have up to six of these stages to achieve high heat recuperation efficiency, and thereby lower the energy demand. This staged approach of mixing and demixing approximates a counter flow heat exchanger in which the temperature of the solids is raised and that of the gas is lowered. As the temperature rises during these pre-heating stages, the calcination reaction of limestone will proceed to an extent that the CO2 partial pressure is not higher than the equilibrium pressure of the calcination reaction. Up to about 30% reaction may be achieved in the pre-heater cyclone stack.
The accumulated pressure drops in each cyclone stage is high as the particles are accelerated in each stage. These pressure drops accumulate and present a significant energy penalty for operating the blowers for forcing the flue gas into the Pre-heater cyclone stack and for drawing out the flue gas.
In the conventional approach, the powder from the pre-heater cyclone stack is injected into a flash calciner where they are mixed with the hot flue gas steam from the clinker kiln described and coal. The hot flue gas stream has excess air which combusts with the coal to drive the calcination reaction towards completion such that 95% calcination is achieved and the exhaust temperature is about 900° C. The exhaust gas temperature is held below that at which the sand will begin to vitrify and calcium silicates begin to form. The solids are once again separated from the flue gas stream, adding to the pressure drop penalty. The pre-heated homogenously mixed lime and sand powders are ready for processing in the rotary kiln.
It will be appreciated that the conventional approach uses powder-gas mixing for each of the stages in the pre-heater stack and in the flash calciner. This gives very efficient heat transfer, but has the undesirable attributes of requiring many stages of powder-gas mixing and separation to achieve an over-all system thermal efficiency.
A disadvantage of the powder-gas mixing is that the exhaust flue gas may have large amounts of cement dust that needs to be separated, and re-injected into the process in order to meet emissions standards. The cost of the filter units is scaled to the gas flow, and the wear of the filter units is associated with the entrained powder. These are disadvantages of the conventional process.
The production of lime is generally carried out in kilns, which are not amenable to CO2 capture described in the present disclosure. However, ground limestone, or lime kiln dust, is calcined in flash calciners similar to that described above for Portland cement. In that case, the pre-heater stack and the flash calciner are augmented by a cyclone cooler which is used to pre-heat the air for combustion. It will be understood by a person skilled in the art that the benefits described in this invention in detail for Portland cement are also applicable with respect to lime production with CO2 capture.
In the case of Portland cement, the pre-heated calcined hot particles are injected into the clinker kiln, which is a rotating kiln fired by a counter-current of flue gas produced from the combustion of coal to a temperature of about 1450° C. with pre-heated air. At these temperatures, the sand fuses with the lime and the particles begin to agglomerate into granules in much the same way as in silica glass manufacturing. In the granules, the reactions proceed to form the calcium silicates that define the composition of Portland cement and the granules sinter. The fusion, reaction and sintering lead to an exhaust stream of calcium silicates in the form of clinker granules of about 10-30 mm in diameter. The clinker granules are cooled by a forced air pre-heater, and then ground to form cement powder. The heated air is used in the combustion process described above. The amount of pre-heated air is sufficient to completely combust the fuel in the rotary kiln and in the flash calciner. This is a large volume of gas that flows counter to the input particles and the growing granules, and the propensity of the lighter particles and granules to be entrained in the gas stream requires a careful design of the rotary kiln.
The rotary kiln flue gas also contains volatile impurities, and an advantage of the mixing in the pre-heater cyclone stack is that these impurities, principally sulphur oxides, react with the raw feed and are oxidised to gypsum, and sequestered in the cement.
The flue gas stream exhausting from the pre-heater cyclone stack is the result of the first and second combustion processes, and contains the carbon dioxide (CO2) from the calcination process. This gas stream has a propensity to comprise a significant amount of carbon monoxide generated in the combustion of the fuels in the presence of the CO2. Carbon monoxide is toxic, and its emissions are regulated. The energy efficiency of the Portland cement process is reduced by the excess air that has to be injected into the combustion process and heated by it. The presence of the CO2 from limestone calcination is a disadvantage of the process.